1. Field of the Invention
The present invention relates to latching systems for telescopic booms of the type comprising inward and outward telescopic boom sections. Specifically, the present invention relates to latching systems for telescopic booms of the type intended to provide an emergency stop in the event of actuator malfunction.
2. The Prior Art
Aerial work platforms of the general type to which the present invention is directed is disclosed in U.S. Pat. No. 3,776,367, and comprise a telescopic boom to the remote end of which a workmans platform is attached. The telescopic boom consists of progressively smaller dimensional boom sections receivable into each other, and the boom sections are hydraulically extended and retracted under the control of the operator. Specifically, as set forth in pending U.S. patent application Ser. No. 165,627, incorporated herein by reference, the telescopic boom sections are actuated by operation of parallel cables or chains, which are anchored at the base of the unit. These extension and retraction chains operate to extend and retract the boom.
An inherent danger with such a piece of equipment is the risk of the chains or cables becoming damaged and breaking. Such an event would cause the boom to retract in an uncontrolled fashion, creating the risk of harm to the operators positioned in the work basket. Further, the booms of such machines are designed to be capable of operation below the plane of the chassis wheels if necessary, that is, to be operated in such a manner so as to extend downward below ground level. There is therefore an additional danger in that a breakage of the chains or cables would under such use cause the boom to extend in an uncontrolled manner, thereby also risking injury to the operator.
Accordingly, manufacturers within the mobile aerial work platform industry have long been in need of a safety mechanism which in the event of cable or chain breakage, would safely and reliably inhibit retraction or extension of the boom. A common approach taken by those desiring to solve this problem has been to incorporate an electrical safety system into aerial work platforms. Typically, such systems use a latch which falls into boom notches continuously unless countered by a solenoid controlled force which is attended by the operators controls. Electrical actuation of the extension/retraction controls by the operator electrically actuates the solenoid and moves the latch away to permit boom movement. While these electrical systems have been effective to some extent, they are deficient in that they do not directly sense the status of the extension/retraction chain but rely upon the electrical integrity of the system. In the event that there is an electrical malfunction in the solenoid system or in the operators controls, the boom would be locked into an extended position, thereby stranding the operator and creating a potentially dangerous condition. Therefore, the electrical systems heretofore presented by the industry have failed to provide a dependable and effective means for sensing the condition of extension and retraction chains, and operating to disable further extension or retraction of the boom upon their breakage.